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Rain water and throughfall chemistry in the silent valley forest in south India
Atmospheric EnmronraentVol 29, No 16, pp 2025-2029, 1995 Copyright © 1995 Elsevier Soence Lid Printed m Great Bntam All rights reserved 1352-2310/95 $9.50 + 0.00
Pergamon
1352-2310(94) 00294-0
RAIN WATER AND THROUGHFALL CHEMISTRY IN THE SILENT VALLEY FOREST IN SOUTH INDIA P. S. P R A K A S A R A O , G. A. M O M I N , P. D. S A F A I , A. G. P I L L A I a n d L. T. K H E M A N I Indian Institute of Tropical Meteorology, Pune 411 008, India (First received 10 February 1993 and m final forrn 1 July 1994) Abstract--Rain water and throughfall samples were collected at the Silent Valley forest dunng the two consecutive southwest summer monsoon seasons (June-September) of 1989 and 1990. All the samples were analysed fol CI-, SO2-, NO~, NH~, Na +, K +, Ca 2+, Mg 2+ and pH. The average pH of rain water was 5.31 and that of throughfall was 6.38 indicating that the throughfall is 10 times more alkaline than the rain water. The buffenng of H + in throughfall was due to interaction with canopy. It was found that the ionic concentrations in throughfall were higher than those In rain water, except for NH~ Enrichment of ionic components in throughfall IS attributed to the Interaction of precipitation wRh the forest canopy. Key word index: Acid rain, atmospheric chemistry, canopy leaching, deposition, ionic fluxes
I. INTRODUCTION To understand the possible impact of atmospheric constituents on ecosystems and to estimate the involved flux rates, il is necessary to know the pathways through which the constituents interact with the ecosystem. Input of acridity by rain to aquatic and terrestrial ecosystems has gained considerable public and scientific interest. Adequate attention has been paid during the last decade to the deposition of air pollutants and more specifically, of trace metals which might threaten the biosphere (Galloway et al., 1982). Pollutant deposition on plant material is a possible cause of the forest disease observed in many parts of Europe and North America (Georgii and Pankrath, 1982; Ulrich and Pankrath, 1983). Such forest diseases cause forest decline and hence lead to local climate change. Dry deposition in forests can be studied by sampling water dripping from canopies during rainfall (throughfall). The composition of throughfall strongly depends on the amount of dry deposition accumulated on the canopies during dry periods. Throughfall quality is also influenced by canopy exchange processes with rain water. Several studies on rain water and throughfall chemistry have been made in various forests in different countries (Forti and Moreira-Nordemann, 1991; Percy, 1989; Mahendrappa, 1989; Coenen et al., 1987; Lindberg et al., 1986). Such studies in forest regions in India are not made so far. Therefore, to study the influence of forest canopy on rain water chemistry, rain water samples were collected in open area and below the canopy during the monsoon seasons of 1989 and 1990 at the
"Silent Valley" a core zone of the Nilgiri Biosphere Reserve (NBR) forest located in Kerala State in South India. 2. SAMPLING SITE
Precipitation samples were collected at the Silent Valley (11°8'N, 76°26'E, 990 m asl). The Silent Valley forests are located at the southwest corner of the Nilgiris, in the western ghat region of peninsular India. It acquired the name "Silent Valley" due to relative absence of the "Cicada" insects which normally makes a distinctive sound in the forest environment. The total extent of the Silent Valley forest is 8952 ha. This is the least disturbed and one of the precious tracts of tropical evergreen forests left in peninsular India. 2.1. Sampling and analysis Rain water samples were collected at two places, one in open area (above the canopy) and another below the canopy, during the two consecutive monsoon seasons (June-September) of 1989-1990. Samples were collected with a collector consisting of a stainless steel funnel (collecting area of 707 cm2), covered with a 2 mm nylon screen mesh) to avoid the deposition of dry leaves, etc. connected to a 2 : polythdene bottle. The funnels were kept 1 m above the ground level at both of the places. The duration of each sample was 24 h. The funnels were washed twice daily in the morning and evening to avoid dry deposition. However, contamination due to dry deposition cannot be completely ruled out. The total number of
2025
2026
P S. PRAKASA RAO et al.
samples in open area during 1989 and 1990 were 57 and 38, respectively, whereas, under the canopy the number of samples were 9 and 33 during 1989 and 1990, respectwely. All the rain water samples were filtered through Whatman 41 filter paper and refrigerated at 4°C until all the ionic components were analysed. The samples were analysed for major ionic components (CI-, SO 2-, NO~, N H I , Na ÷, K ÷, Ca 2÷ and Mg 2÷) by using standard analytical methods described elsewhere (Khemani et al., 1985). The pH of unfiltered samples was measured using a digital pH meter.
8
--
,
Throughfall\!~
•
I
I
7
5
I
I
I
I
I
I
I
I
I
I
I
I
I
~ ' ~ l i 0 1 32t2' 12 95 2 8 3 1 3 41963 7 4 0 4 3 ~
I
"i
1989
Fig. 1 pH values m rain water ( ) and throughfall (. . . . . ) in the Silent Valley forests
3. R E S U L T S A N D D I S C U S S I O N
3.1.
pH o f rain water and throuohfall
The reference level commonly used to compare acidic precipitation with natural precipitation is pH 5.6; the pH that results from the equilibrium of atmospheric CO2 with precipitation. The average pH of rain water (open area) during the monsoon season of 1989 was 6.19, whereas it was 5.04 during the monsoon season of 1990 (Table 1). The pH decreased by 1.1 units during 1990. Out of 38 samples during 1990, 8 samples showed pH < 5.0. However during the period of 1989, none of the sample was observed to be < 5.0 pH. This variation in the pH values during the two seasons might be due to variation in the concentrations of organic acids (formic and acetic). However, the variation in pH values could have been better explained if measurements of orgamc acids would have been made. It is reported that pH of rain water in unpolluted areas can be around 5 due to the presence of naturally produced organic acads like formic and acetic acids (Galloway et al., 1982). However, the pH of throughfall (below canopy) varied between 5.22 and 7.65 with an average of 6.38 which is alkaline. Hence, the throughfall is 10 times more alkaline than the rain water. The pH values in throughfall on 9 occasions in 1989 and on 33 occasions in 1990 and their corresponding values in rain water are given in Fig. 1. It can be seen from this figure that pH values in throughfall samples are, by and large, higher than those in rain water samples during both the monsoon seasons. The
higher pH in throughfall is due to the washout of the soil-oriented ions deposited on the leaves and the alkaline ionic emissions by the leaves. H ÷ ion loss and pH increase in throughfaU is generally believed to be due to ion exchange of H ÷ with other cations on canopy exchange sites. Also, higher pH was reported in throughfall precipitation than in bulk precipitation collected at two ridge top appalachian deciduous forest sites m U.S.A. (Dewalle et al., 1988). Bredemeier (1988) reported that during the leafless period buffering rates are very low compared to the leafy period when elevated pH values were observed in throughfall samples. It is of interest to note that the silent valley Is free from industrial pollution and the level of total suspended particulates (TSP) was reported to be around 37/~gm -3. In other parts of the country, the TSP levels have varied from 200 to 600 #g m - 3 (Safai et al., 1993). The average pH (5.31) in ram water is slightly acidic since the contribution of acidic ions (SO 2- and NO~) are quite low. The precipitation acidity originates primarily from sulphuric and nitric acids and the original acidity is conserved in the concentrations of SO 2- and NO~. The concentration of SO 2- is around 1 mg E- 1 which is in the range of background level. Majority of NO~ aerosols in the silent valley are
Table 1. Average lomc composition and pH of rain water and throughfall at Silent Valley during the monsoon seasons of 1989 and 1990 Concentration 0zeq~- 1) Year Rain water
Throughfall
1989 1990 Mean 1989 1990 Mean
No. of samples 57 38 9 33
Cl- SO42- NO~- NH~ Na +
K + Ca 2+ Mg 2+ H +
37 48 43 80 81 81
3 4 4 75 47 51
21 19 20 25 19 22
11 30 21 25 43 34
2 3 3 3 3 3
41 51 46 71 90 81
46 40 43 93 88 91
14 14 14 32 42 37
0.65 9.12 4.89 0.19 0.65 0.42
Rain water and throughfall chemistry contributed by the soil. Safai et al. (1993) reported around 70% of the mass of total NO~ aerosol in the coarse mode ( > :!.5 #m dia.) in the Silent Valley. However, around 73% of SO~- was reported in the submicron mode ( < 2.5 tim dia.). This suggests that acidity in rain water is caused by H2SO4 rather than HNO3. Also, the .average pH of 5.31 in rain water might be due to the influence of organic acids (formic and acetic) which were not measured. The average pH of rain water collected above the canopy in the present study is in agreement with the reported pH values (pH = 5.3) for a tropical rain forest in Coasta Rica (Hendry et al., 1984). The rain water in tropical rain forests is acidic with values between 4.5 and 5.0 (Rodhe et al., 1988).
3.2. Ionic composition The quality of ]precipitation falling on the forest is altered during a brief but significant interaction with the surfaces of the plants, resulting in a transfer of additional mineral matter to the forest floor. Throughfall and stem flow are the major pathways in the nutrient cycling. As stem flow transfers only 5-20% of the total nutrients in rain water (Brinkmann and Dos Santos, 1971), it was not studied in this work. The average ionic composition of rain water and throughfall collected for the two consecutive monsoon seasons of 1989 and 1990 along with pH values are given in Table 1. The average concentrations of all the ionic components in throughfall were considerably higher than those in rain water except NH~ and H +. Among all ions, K + and Mg 2+ were substantially increased in throughfall. Dewalle et al. (1988) have also reported the highest enrichment of K ÷ in throughfall collected at two appalachian deciduous forests in U.S.A.
2027
The net concentration ratios (NCR) were calculated in throughfall and given in Table 2 by following method. NCR = Throughfall c o n e . - Rain water cone. Rain water cone. The net concentration ratios in throughfall samples indicate that canopy interactions caused reduction in the concentration of H + and enhancement in concentrations of all other major ions except NH~. In the case of NH~ no change was observed due to canopy interactions. Throughfall is a very important source of K ÷ and Mg 2+ for the soil. Of the total ionic input reaching the soil, about 8% K and 38% Mg were contributed by rain water. The remaining was added to precipitation by throughfall. These elements participate in the recycling processes in the forest. Garten et al. (1988) reported that the canopy leaching may be important for several cations (K +, Ca 2 + and Mg 2+). The major fraction of SOl,- ion in tbroughfall is from rain water indicating that only 10% enrichment of S O l - is from throughfall in the present study. Canopy exchange processes have lesser effect on the concentrations of SO~,-, Na ÷ and C1- in throughfall (Garten et al., 1988). The high concentrations of sea salt (C1- and Na ÷) in throughfall in the present study may be due to the monsoon currents which carry these particles from the sea and deposit on the forest canopy. The 110% enrichment of Ca 2 ÷ in throughfall may be due to removal of the dust particles deposited on the leaves and leaching from the leaves. 3 3. Input fluxes
The seasonal ionic input fluxes (kg h a - t season- 1) for various components by rain and throughfall were calculated and given in Table 3. It can be seen from
Table 2. Mean seasonal (June-September) net concentration ratios for throughfall at Sdent Valley forest Ion Net concentration RaUo (%)
CI-
SO~-
NO~
NH~"
Na +
K+
Ca 2+
Mg 2+
H+
88
10
62
0
76
1175
I10
164
- 91
Table 3. Seasonal (June-September) input fluxes of various ionic components through ram water and throughfall at Silent Valley Input fluxes (kg ha- season- 1) No. of
Rain water
Throughfall
Year
~amples
CI-
SO,2-
NO~
NH4+
Na +
K*
Ca 2+
Mg 2÷
1989 1990 Mean 1989 1990 Mean
57 38
48.8 58.0 53.4 106.5 98.8 102.7
37.5 31.2 34 4 45.8 31.6 38.7
26.1 64.1 45.2 59.3 91 9 75.6
1.1 1.4 13 2.3 17 20
35.6 36.4 36.0 61.1 71.0 66 0
4.1 48 45 75.8 65.9 70 9
34.1 27.4 30.8 69.8 60.0 64.9
6.4 5.8 61 14.3 17 2 15 8
9 33
2028
P.S. PRAKASA RAO et al.
Table4 Input fluxes (kgha-lseason -1) of Ca2+, K +, Mg2+ and Na + through ram, throughfall, mterceptmn deposmon and canopy leaching Ionic species Ca 2+ K+ Mg2+ Na ÷
Ram water
Throughfall
Interceptmn" depositmn
Canopy leaching
30.8 4.5 6.1 36.0
64.9 70.9 15.8 66.0
25.7 3.8 5.1 30.0
8.4 62.7 4.6 Nil
~Calc~a~d. the table that the input fluxes of all the ionic components to the forest soil were more from throughfall than those from rain. The ionic return by rain water had the following sequence: CI- 1 > NO~ > N a + > S O 2 - > C a 2+ > M g z+ > K + > N H 2 . Whereas, by throughfall, it was: CI- > N O r > K + > Na + > Ca z+ > SO [ - > Mg a+ > NH2. This feature indicates that the above two sequences followed the same pattern except for K +. This may be due to high enrichment of K + in throughfall because of its emission and leaching from the vegetation. While discussing the mass size distribution of K + aerosols in the Silent Valley, Safai et al. (1993) reported that majority of K + aerosols are released by vegetation through leaves by guttation processes. The enrichment of ionic components in throughfall compared to precipitation is caused by interception deposition (impaction of aerosols plus deposition of gases) as well as release and uptake of substances by the canopy. Neither interception deposition nor canopy leaching can be easily quantified, but for elements without gaseous components (e.g. Na +, Ca 2 +, Mg 2+ and K+); the respective contributions can be separated assuming that these elements show the same ratio between precipitation deposition and interception deposition for each individual sample (Ulrich, 1983). This ratio can be estimated using Na +, which is generally believed not to be noticeably affected by canopy exchange, except for some minor leaching from scenescent needles (Ulrich, 1983). The Na + enrichment in throughfall is hence interpreted solely as downwash of particles deposited on the canopy. By using this assumption, the ionic fluxes of Ca z+, K +, Mg 2+ and Na + through interception deposition and canopy leaching were calculated and given in Table 4. It can be seen from Table 4 that the Ca 2 +, Mg 2 + and K + are leached from the canopy. The percentage leaching of Ca z +, Mg z + and K + from the canopy was 13%, 29% and 88%, respectively. The maximum input flux of K + is through canopy leaching since K + being the most biologically active cation (Freiesleben et al., 1986). The canopy leaching of Ca z+ was found minimum. 4. CONCLUSIONS The rain water at the Silent Valley was, by and large, slightly acidic (pH = 5.31) and near the CO2-
equilibrated value. The ionic concentratmns were higher in throughfall than in rain water due to the interaction of rain water with vegetation. The major elements transferred from the vegetation to the throughfall were K + (92%), and Mg 2+ (62%) It may be inferred that the main source of K + and Mg 2 + m the throughfall was from the washout of the depositions on the vegetation, and leaching from the vegetation. The S O l - enrichment in throughfall is only 10%, indicating that the canopy interaction does not contribute any S O l - . Greater exchangeable basic conditions gave rise to more reduction m H + as water moved through the canopy to the forest floor. Acknowledgements--Authors are thankful to Ministry of En-
wronment and Forests, Govt. of India, for providing financial support for undertaking the field programmes in NBR. Authors are also thankful to Dr. E. J. James, CWRDM, Calicut and Divisional Forest Officer, Kerala State Forest Department for their support and valuable co-operation in conducting the field observations at the Silent Valley.
REFERENCES
Bredemeier M. (1988) Forest Canopy Transformation of Atmospheric Deposition. Wat. Azr Soil PoUut. 40, 121-138. Brinkmann W. L. F. and Dos Santos A. (1971) Natural waters in Amazonia. V. Soluble magnesium properties. Turrialba 21, 459-465. Coenen B., Ronneau C. and Cara J. (1987) AccumulaUonof mr pollutants by a spruce forest in eastern Belgium. Wat Air Soil Pollut. 36, 231-237 Dewalle D. R., Sharpe W. E. and Edwards P. J. (1988) Biogeochemistryof two appalachian deciduous forest sites in relation to episodic stream acidification. Wat. Air Soil Pollut. 40, 143-156. Forti M. C. and Morcira-Nordemann L M. (1991) Rain water and throughfaU chemistry in a "Tera Firme" rain forest: Central Amazonia. J. oeophys. Res. 96, 7415-7421. Freieslebcn N. E. V., Ridder C and Rasmussen L (1986) Patterns of acid precipitation to a Danish spruce forest. War. Air Soil Pollut. 30, 135-141. Galloway J. N., Likens G. E., Kcene W. C. and Miller J. M. (1982) The composition of prcopitation m remote areas of the world. J. geophys. Res. 87, 8771-8786. Garten C. T., Bon&etti E. A. and Lomax R. D. (1988) Contribution of foliar leaching and dry deposition to sulphate in net throughfall below deciduous trees. Atmospheric Environment 22, 1425-1432. Georgii H. W. and Pankrath J. (eds) (1982) Deposition of Atmospheric Pollutants. D. Reidel, Dordrecht. Hendry D. C., Bcrish C. W. and Edgerton E. S. (1984) Precipitation chemistry at Turrialba, Coasta Rica, Wat. Resource Res. 20, 1677-1684. Khemani L. T., Momin G. A., Naik M. S., Rao P. S. P.,
Kumar R. and Ramana Murty Bh. V. (1985) Impact of alkaline particulates on pH of rain water in India. War. Air Sod PoUut. 25, 365-376. Lindbcrg S. E., Lovett G. M, Richter D R. and Johnston D. W. (1986) Atmospheric deposition and canopy interaction of major ions m a forest. Science 231, 141-145. Mahendrappa M. K. (1989) Impacts of forests on water chemistry. Wat. Air Soil Pollut. 46, 61-72, Percy K. E. (1989)Vegetation, soils and ion transfer through the forest canopy in two Nova Scotia lake basins. Wat. A,r Soil Pollut. 46, 73-86.
Rain water and throughfall chemistry Rodhe H., Cowling E., Galbally I. E., Galloway J. N. and Herrera R. (1988) Acidification and regional air pollution in the tropics. In Acidification in Tropical Countries (edited by Rodhe H. and Herrera R.), pp. 3-39. Wiley, New York. Safal P. D., Khemani L. T., Momin (3. A., Rao P. S. P. and Pillai A. (3. (1993.) Mass size distribution and chemical composmon of aerosol at the Silent Valley, India. In&an J. Radio Space Phys. 22, 56-61.
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Ulrich B (1993) Interaction of forest canopies with atmospheric constituents; SOz, alkali and earth alkali cations and chloride. In Effects of Accumulation of Air Pollutants in Forest Ecosystems (edited by Ulrich B and Pankrath J.), pp. 33-45. D. Rcldel, Dordrecht. Ulnch B. and Pankrath J. (eds) (1983) Effects of Accummulatwn of Azr Pollutants in Forest Ecosystem. D. Reidel, Dordrecht.
Pergamon
1352-2310(94) 00294-0
RAIN WATER AND THROUGHFALL CHEMISTRY IN THE SILENT VALLEY FOREST IN SOUTH INDIA P. S. P R A K A S A R A O , G. A. M O M I N , P. D. S A F A I , A. G. P I L L A I a n d L. T. K H E M A N I Indian Institute of Tropical Meteorology, Pune 411 008, India (First received 10 February 1993 and m final forrn 1 July 1994) Abstract--Rain water and throughfall samples were collected at the Silent Valley forest dunng the two consecutive southwest summer monsoon seasons (June-September) of 1989 and 1990. All the samples were analysed fol CI-, SO2-, NO~, NH~, Na +, K +, Ca 2+, Mg 2+ and pH. The average pH of rain water was 5.31 and that of throughfall was 6.38 indicating that the throughfall is 10 times more alkaline than the rain water. The buffenng of H + in throughfall was due to interaction with canopy. It was found that the ionic concentrations in throughfall were higher than those In rain water, except for NH~ Enrichment of ionic components in throughfall IS attributed to the Interaction of precipitation wRh the forest canopy. Key word index: Acid rain, atmospheric chemistry, canopy leaching, deposition, ionic fluxes
I. INTRODUCTION To understand the possible impact of atmospheric constituents on ecosystems and to estimate the involved flux rates, il is necessary to know the pathways through which the constituents interact with the ecosystem. Input of acridity by rain to aquatic and terrestrial ecosystems has gained considerable public and scientific interest. Adequate attention has been paid during the last decade to the deposition of air pollutants and more specifically, of trace metals which might threaten the biosphere (Galloway et al., 1982). Pollutant deposition on plant material is a possible cause of the forest disease observed in many parts of Europe and North America (Georgii and Pankrath, 1982; Ulrich and Pankrath, 1983). Such forest diseases cause forest decline and hence lead to local climate change. Dry deposition in forests can be studied by sampling water dripping from canopies during rainfall (throughfall). The composition of throughfall strongly depends on the amount of dry deposition accumulated on the canopies during dry periods. Throughfall quality is also influenced by canopy exchange processes with rain water. Several studies on rain water and throughfall chemistry have been made in various forests in different countries (Forti and Moreira-Nordemann, 1991; Percy, 1989; Mahendrappa, 1989; Coenen et al., 1987; Lindberg et al., 1986). Such studies in forest regions in India are not made so far. Therefore, to study the influence of forest canopy on rain water chemistry, rain water samples were collected in open area and below the canopy during the monsoon seasons of 1989 and 1990 at the
"Silent Valley" a core zone of the Nilgiri Biosphere Reserve (NBR) forest located in Kerala State in South India. 2. SAMPLING SITE
Precipitation samples were collected at the Silent Valley (11°8'N, 76°26'E, 990 m asl). The Silent Valley forests are located at the southwest corner of the Nilgiris, in the western ghat region of peninsular India. It acquired the name "Silent Valley" due to relative absence of the "Cicada" insects which normally makes a distinctive sound in the forest environment. The total extent of the Silent Valley forest is 8952 ha. This is the least disturbed and one of the precious tracts of tropical evergreen forests left in peninsular India. 2.1. Sampling and analysis Rain water samples were collected at two places, one in open area (above the canopy) and another below the canopy, during the two consecutive monsoon seasons (June-September) of 1989-1990. Samples were collected with a collector consisting of a stainless steel funnel (collecting area of 707 cm2), covered with a 2 mm nylon screen mesh) to avoid the deposition of dry leaves, etc. connected to a 2 : polythdene bottle. The funnels were kept 1 m above the ground level at both of the places. The duration of each sample was 24 h. The funnels were washed twice daily in the morning and evening to avoid dry deposition. However, contamination due to dry deposition cannot be completely ruled out. The total number of
2025
2026
P S. PRAKASA RAO et al.
samples in open area during 1989 and 1990 were 57 and 38, respectively, whereas, under the canopy the number of samples were 9 and 33 during 1989 and 1990, respectwely. All the rain water samples were filtered through Whatman 41 filter paper and refrigerated at 4°C until all the ionic components were analysed. The samples were analysed for major ionic components (CI-, SO 2-, NO~, N H I , Na ÷, K ÷, Ca 2÷ and Mg 2÷) by using standard analytical methods described elsewhere (Khemani et al., 1985). The pH of unfiltered samples was measured using a digital pH meter.
8
--
,
Throughfall\!~
•
I
I
7
5
I
I
I
I
I
I
I
I
I
I
I
I
I
~ ' ~ l i 0 1 32t2' 12 95 2 8 3 1 3 41963 7 4 0 4 3 ~
I
"i
1989
Fig. 1 pH values m rain water ( ) and throughfall (. . . . . ) in the Silent Valley forests
3. R E S U L T S A N D D I S C U S S I O N
3.1.
pH o f rain water and throuohfall
The reference level commonly used to compare acidic precipitation with natural precipitation is pH 5.6; the pH that results from the equilibrium of atmospheric CO2 with precipitation. The average pH of rain water (open area) during the monsoon season of 1989 was 6.19, whereas it was 5.04 during the monsoon season of 1990 (Table 1). The pH decreased by 1.1 units during 1990. Out of 38 samples during 1990, 8 samples showed pH < 5.0. However during the period of 1989, none of the sample was observed to be < 5.0 pH. This variation in the pH values during the two seasons might be due to variation in the concentrations of organic acids (formic and acetic). However, the variation in pH values could have been better explained if measurements of orgamc acids would have been made. It is reported that pH of rain water in unpolluted areas can be around 5 due to the presence of naturally produced organic acads like formic and acetic acids (Galloway et al., 1982). However, the pH of throughfall (below canopy) varied between 5.22 and 7.65 with an average of 6.38 which is alkaline. Hence, the throughfall is 10 times more alkaline than the rain water. The pH values in throughfall on 9 occasions in 1989 and on 33 occasions in 1990 and their corresponding values in rain water are given in Fig. 1. It can be seen from this figure that pH values in throughfall samples are, by and large, higher than those in rain water samples during both the monsoon seasons. The
higher pH in throughfall is due to the washout of the soil-oriented ions deposited on the leaves and the alkaline ionic emissions by the leaves. H ÷ ion loss and pH increase in throughfaU is generally believed to be due to ion exchange of H ÷ with other cations on canopy exchange sites. Also, higher pH was reported in throughfall precipitation than in bulk precipitation collected at two ridge top appalachian deciduous forest sites m U.S.A. (Dewalle et al., 1988). Bredemeier (1988) reported that during the leafless period buffering rates are very low compared to the leafy period when elevated pH values were observed in throughfall samples. It is of interest to note that the silent valley Is free from industrial pollution and the level of total suspended particulates (TSP) was reported to be around 37/~gm -3. In other parts of the country, the TSP levels have varied from 200 to 600 #g m - 3 (Safai et al., 1993). The average pH (5.31) in ram water is slightly acidic since the contribution of acidic ions (SO 2- and NO~) are quite low. The precipitation acidity originates primarily from sulphuric and nitric acids and the original acidity is conserved in the concentrations of SO 2- and NO~. The concentration of SO 2- is around 1 mg E- 1 which is in the range of background level. Majority of NO~ aerosols in the silent valley are
Table 1. Average lomc composition and pH of rain water and throughfall at Silent Valley during the monsoon seasons of 1989 and 1990 Concentration 0zeq~- 1) Year Rain water
Throughfall
1989 1990 Mean 1989 1990 Mean
No. of samples 57 38 9 33
Cl- SO42- NO~- NH~ Na +
K + Ca 2+ Mg 2+ H +
37 48 43 80 81 81
3 4 4 75 47 51
21 19 20 25 19 22
11 30 21 25 43 34
2 3 3 3 3 3
41 51 46 71 90 81
46 40 43 93 88 91
14 14 14 32 42 37
0.65 9.12 4.89 0.19 0.65 0.42
Rain water and throughfall chemistry contributed by the soil. Safai et al. (1993) reported around 70% of the mass of total NO~ aerosol in the coarse mode ( > :!.5 #m dia.) in the Silent Valley. However, around 73% of SO~- was reported in the submicron mode ( < 2.5 tim dia.). This suggests that acidity in rain water is caused by H2SO4 rather than HNO3. Also, the .average pH of 5.31 in rain water might be due to the influence of organic acids (formic and acetic) which were not measured. The average pH of rain water collected above the canopy in the present study is in agreement with the reported pH values (pH = 5.3) for a tropical rain forest in Coasta Rica (Hendry et al., 1984). The rain water in tropical rain forests is acidic with values between 4.5 and 5.0 (Rodhe et al., 1988).
3.2. Ionic composition The quality of ]precipitation falling on the forest is altered during a brief but significant interaction with the surfaces of the plants, resulting in a transfer of additional mineral matter to the forest floor. Throughfall and stem flow are the major pathways in the nutrient cycling. As stem flow transfers only 5-20% of the total nutrients in rain water (Brinkmann and Dos Santos, 1971), it was not studied in this work. The average ionic composition of rain water and throughfall collected for the two consecutive monsoon seasons of 1989 and 1990 along with pH values are given in Table 1. The average concentrations of all the ionic components in throughfall were considerably higher than those in rain water except NH~ and H +. Among all ions, K + and Mg 2+ were substantially increased in throughfall. Dewalle et al. (1988) have also reported the highest enrichment of K ÷ in throughfall collected at two appalachian deciduous forests in U.S.A.
2027
The net concentration ratios (NCR) were calculated in throughfall and given in Table 2 by following method. NCR = Throughfall c o n e . - Rain water cone. Rain water cone. The net concentration ratios in throughfall samples indicate that canopy interactions caused reduction in the concentration of H + and enhancement in concentrations of all other major ions except NH~. In the case of NH~ no change was observed due to canopy interactions. Throughfall is a very important source of K ÷ and Mg 2+ for the soil. Of the total ionic input reaching the soil, about 8% K and 38% Mg were contributed by rain water. The remaining was added to precipitation by throughfall. These elements participate in the recycling processes in the forest. Garten et al. (1988) reported that the canopy leaching may be important for several cations (K +, Ca 2 + and Mg 2+). The major fraction of SOl,- ion in tbroughfall is from rain water indicating that only 10% enrichment of S O l - is from throughfall in the present study. Canopy exchange processes have lesser effect on the concentrations of SO~,-, Na ÷ and C1- in throughfall (Garten et al., 1988). The high concentrations of sea salt (C1- and Na ÷) in throughfall in the present study may be due to the monsoon currents which carry these particles from the sea and deposit on the forest canopy. The 110% enrichment of Ca 2 ÷ in throughfall may be due to removal of the dust particles deposited on the leaves and leaching from the leaves. 3 3. Input fluxes
The seasonal ionic input fluxes (kg h a - t season- 1) for various components by rain and throughfall were calculated and given in Table 3. It can be seen from
Table 2. Mean seasonal (June-September) net concentration ratios for throughfall at Sdent Valley forest Ion Net concentration RaUo (%)
CI-
SO~-
NO~
NH~"
Na +
K+
Ca 2+
Mg 2+
H+
88
10
62
0
76
1175
I10
164
- 91
Table 3. Seasonal (June-September) input fluxes of various ionic components through ram water and throughfall at Silent Valley Input fluxes (kg ha- season- 1) No. of
Rain water
Throughfall
Year
~amples
CI-
SO,2-
NO~
NH4+
Na +
K*
Ca 2+
Mg 2÷
1989 1990 Mean 1989 1990 Mean
57 38
48.8 58.0 53.4 106.5 98.8 102.7
37.5 31.2 34 4 45.8 31.6 38.7
26.1 64.1 45.2 59.3 91 9 75.6
1.1 1.4 13 2.3 17 20
35.6 36.4 36.0 61.1 71.0 66 0
4.1 48 45 75.8 65.9 70 9
34.1 27.4 30.8 69.8 60.0 64.9
6.4 5.8 61 14.3 17 2 15 8
9 33
2028
P.S. PRAKASA RAO et al.
Table4 Input fluxes (kgha-lseason -1) of Ca2+, K +, Mg2+ and Na + through ram, throughfall, mterceptmn deposmon and canopy leaching Ionic species Ca 2+ K+ Mg2+ Na ÷
Ram water
Throughfall
Interceptmn" depositmn
Canopy leaching
30.8 4.5 6.1 36.0
64.9 70.9 15.8 66.0
25.7 3.8 5.1 30.0
8.4 62.7 4.6 Nil
~Calc~a~d. the table that the input fluxes of all the ionic components to the forest soil were more from throughfall than those from rain. The ionic return by rain water had the following sequence: CI- 1 > NO~ > N a + > S O 2 - > C a 2+ > M g z+ > K + > N H 2 . Whereas, by throughfall, it was: CI- > N O r > K + > Na + > Ca z+ > SO [ - > Mg a+ > NH2. This feature indicates that the above two sequences followed the same pattern except for K +. This may be due to high enrichment of K + in throughfall because of its emission and leaching from the vegetation. While discussing the mass size distribution of K + aerosols in the Silent Valley, Safai et al. (1993) reported that majority of K + aerosols are released by vegetation through leaves by guttation processes. The enrichment of ionic components in throughfall compared to precipitation is caused by interception deposition (impaction of aerosols plus deposition of gases) as well as release and uptake of substances by the canopy. Neither interception deposition nor canopy leaching can be easily quantified, but for elements without gaseous components (e.g. Na +, Ca 2 +, Mg 2+ and K+); the respective contributions can be separated assuming that these elements show the same ratio between precipitation deposition and interception deposition for each individual sample (Ulrich, 1983). This ratio can be estimated using Na +, which is generally believed not to be noticeably affected by canopy exchange, except for some minor leaching from scenescent needles (Ulrich, 1983). The Na + enrichment in throughfall is hence interpreted solely as downwash of particles deposited on the canopy. By using this assumption, the ionic fluxes of Ca z+, K +, Mg 2+ and Na + through interception deposition and canopy leaching were calculated and given in Table 4. It can be seen from Table 4 that the Ca 2 +, Mg 2 + and K + are leached from the canopy. The percentage leaching of Ca z +, Mg z + and K + from the canopy was 13%, 29% and 88%, respectively. The maximum input flux of K + is through canopy leaching since K + being the most biologically active cation (Freiesleben et al., 1986). The canopy leaching of Ca z+ was found minimum. 4. CONCLUSIONS The rain water at the Silent Valley was, by and large, slightly acidic (pH = 5.31) and near the CO2-
equilibrated value. The ionic concentratmns were higher in throughfall than in rain water due to the interaction of rain water with vegetation. The major elements transferred from the vegetation to the throughfall were K + (92%), and Mg 2+ (62%) It may be inferred that the main source of K + and Mg 2 + m the throughfall was from the washout of the depositions on the vegetation, and leaching from the vegetation. The S O l - enrichment in throughfall is only 10%, indicating that the canopy interaction does not contribute any S O l - . Greater exchangeable basic conditions gave rise to more reduction m H + as water moved through the canopy to the forest floor. Acknowledgements--Authors are thankful to Ministry of En-
wronment and Forests, Govt. of India, for providing financial support for undertaking the field programmes in NBR. Authors are also thankful to Dr. E. J. James, CWRDM, Calicut and Divisional Forest Officer, Kerala State Forest Department for their support and valuable co-operation in conducting the field observations at the Silent Valley.
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